In 1984, Muhammad Ali was a 42 years old former boxing champion who recently had to figure out how to live with his tremors, slurred speech and slow movements. He was diagnosed with Parkinson’s disease.

Ali fought late into his life; and especially his last two fights, Holmes (above) and Berbick, left him battered and bruised… to say the least.

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There are many people that believe Ali would have developed PD even if he lived a life as a quiet librarian, since the evidence linking boxing-related head trauma to development of PD is still controversial.

However, there’s another group that believe in a definitive cause-and-effect relationship.

Another case-controlled study examined environmental risk of PD. This study found that head injury (OR=6.23) that occurred on average 37yrs prior to diagnosis was the strongest risk-factor for PD… Though you have to keep-in-mind people have a hard time remembering these events or may attach greater significance to them in hindsight, i.e.”recall bias”.